In recent years, the technique for TFTs for liquid crystal display devices has progressed, and servicing for information infrastructure has been made satisfactory. Thus, at the present time, the FPD is proposed, and even in the medical image field, the FPD can have a large area and digitization of the FPD is attained.
This FPD is adapted to read out a radiation image in an instant to display the image on a display device simultaneously, and an image can be directly fetched in the form of digital information from the FPD. Thus, the FPD has the feature that handling and management is convenient in the safekeeping of data, and in the processing and transfer of data. In addition, it was verified that though the characteristics such as sensitivity depend on photographing conditions, the characteristics are equal to or superior to those in a conventional screen film photographing method or a computed radiography photographing method.
Commercialization of the FPD has been attained. On the other hand, various proposals for the FPD have been made for the purpose of aiming at further enhancing the sensitivity. For example, in a paper by L. E. Antonuk et al., in the journal SPIE Medical Imaging VI, February, pp. 23 to 27, 1992, there is disclosed a structure in which a sensor element is formed on a TFT element. In this example, adoption of the above-mentioned structure allows an open area ratio of the sensor element to be increased to make enhancement of sensitivity possible.
In addition, it is described that since the TFT element is disposed right under the sensor element, an unnecessary parasitic capacitance is formed, and hence a grounded plane is provided.
In addition, in a proposal made in U.S. Pat. No. 5,498,880, granted to DuPont, likewise, there is shown a structure in which in order to increase an open area ratio, a sensor element is formed on a TFT element. In this example, there is adopted a structure in which an electrode connected to a source/drain electrode of the TFT covers the TFT element, and also serves as a separate electrode of the sensor element.
On the other hand, in a proposal in Japanese Patent Application Laid-Open No. 2000-156522, filed by Canon Kabushiki Kaisha, there is shown a structure in which for the purpose of aiming at increasing an open area ratio, a sensor element is formed above a TFT element. In this example, there is adopted a structure in which the sensor element is formed over the TFT element, but spaced from the latter by an interlayer film.
However, in the above-mentioned FPD having the sensor element formed on the TFT element, the separate electrode of the sensor element acts as a back gate electrode of the TFT element. Hence, a problem of generation of a leakage current of the TFT element is caused by the fluctuation in electric potential of the separate electrode. Such a problem appears in the form of degradation of quality of the image.
In a case where for example, an area having a large sensor output signal and an area having a small sensor output signal are disposed adjacent to each other, crosstalk that blurs the boundary between these areas appears. In addition, there is caused a problem that sensor saturation output is decreased, reducing the dynamic range.